When both the solar and view zenith direction approach horizontal, the reflected radiance at the top of the atmosphere, derived from the plain-parallel atmospheric radiative transfer equation, is different and it depends on the path how the incidence and reflected directions tend to be horizontal. This phenomenon under extreme condition is called limit discontinuity in mathematics. However, the discontinuity of the limits is contradictory to the physical nature of atmospheric radiative transfer and it is obtained by neglecting an implicit physics principles, the Snell's law. The Snell's law must be used in the radiative transfer algorithm for radiance under this extreme condition to avoid obtaining wrong results.